Inoculation into nude mice of spontaneously transformed Balb 3T3 cells has revealed that tumor development is a problem of cellular sociobiology, ecology and heritable variation. This is manifest in the observations that a minimum of 1000 cells is required to initiate a tumor and many such "low dose" tumors are inordinately slow in development until a burst of rapid growth occurs. The sequence is pathognomonic of tumor progression. By contrast, the innoculation of 10/6 cells uniformly results in rapid and sustained development of tumors. A similar population dependent multiplication of these cells can be produced in culture by using a medium deficient in specific growth factors, e.g., by substituting chicken serum for the conventionally used calf serum. The similarity between the in vivo and in vitro situations has pointed up the likelihood that the interstitial fluid, which is a filtrate of plasma rather than serum, and which serves as the medium for cells inoculated in subcutaneous tissue of the mouse, is even more deficient in growth factors than is plasma. Tumor development, therefore, depends on the microenvironment including cellular interactions and on heritable variation in the cells. Our plan is to study all the factors which contribute to cell multiplication and tumor progression in the easily quantified situation of cell culture. Alternative strategies will be applied for simulating the growth factor deficient environment of interstitial fluid, including the use of plasma, lymphatic fluid and combinations of growth factors used in commercial serum- free defined medium. The nature of the specific deficiency in chicken serum will be elucidated, and the effect of unfavorable growth environments on the frequency of heritable cell variation will be measured. A study will be made of the interactions among cells which permit rapid multiplication of large numbers of cells in a medium deficient in specific growth factors. Any conditioning on autocrine growth factors released by the cells will be characterized. The work in culture will be coordinated with quantitative experiments on tumor development in nude mice. The aim is to sort out the various processes, factors and interactions which contribute to tumor progression and thereby develop a rationale for its eventual control.